Water-dilutable binders for paints and other coating compositions in various forms have been developed in recent years substantially for ecological reasons. Aqueous dispersions of alkyd resins, in the following consistently called “alkyd resin emulsions” used for higher-quality paints, such as topcoats on wood and metal, have, for reasons due to their preparation, their stabilisation or their further processing, a mass fraction of solids of not more than 45% and contents of organic solvents and/or of volatile neutralising agents, such as amines and ammonia.
These contents can indeed be decreased, as described in U.S. Pat. No. 3,269,967 or U.S. Pat. No. 3,440,193, if emulsifiers such as alkylphenol ethoxylates and fatty alcohol ethoxylates are used, but the properties of the films formed, such as drying, hardness, gloss and resistance to water and weather, are adversely influenced by these low molar mass substances, which are present in unchanged form in the cured film. These binders therefore cannot be used for topcoats.
In the Austrian Patent Specifications AT-B 325 742, 333 035 and 336 277, the use of emulsifier resins is described which are modified with polyethylene glycol for emulsification of alkyd resins, as a result of which the disadvantages mentioned are said to be largely avoided. However, to achieve stable emulsions, in addition to a higher content of emulsifier resin, these binders also require neutralising agents for stabilisation and also organic auxiliary solvents. Films obtained with such binder emulsions consequently also show deficiencies in drying and resistance to water and weather.
It has also not been possible to eliminate these disadvantages by the use of unsaturated polyurethanes as emulsifiers, such as are described in the document DE-A 43 06 946.
According to the teaching of U.S. Pat. No. 3,457,206, the use of alkoxypolyethylene glycol (polyethylene glycol monoalkyl ether) for emulsification also leads to aqueous alkyd resins only in combination with neutralisation of free carboxyl groups with alkanolamines or alkali metal hydroxides, but these alkyd resins are not suitable for high-solids paints.
According to the teaching of WO-A 94/18260, water-soluble resins which are used as dispersing agents or wood preservatives are obtained by using alkoxypolyethylene glycol also without neutralisation of acidic groups. These resins contain methoxypolyethylene glycol bonded in a readily hydrolysable form and can be diluted with water only shortly before use. They are not suitable as a sole binder for weather-resistant, glossy coatings such as are expected of alkyd resin topcoats.
In EP-A 1 092 742, it has been described that high-solids, aqueous emulsions of alkyd resins are obtained without additional neutralisation and solvent if the preparation of the emulsions is carried out by using emulsifier resins which have a particular constitution and are based on a polyhydroxy component from the class of sugar alcohols which are capable of ring closure by dehydration, an alkoxypolyethylene glycol and mono- and dicarboxylic acids or anhydrides thereof. Due to the specific constitution of the emulsifier resin and the exclusive use of cycloaliphatic dicarboxylic acids, a sufficiently high resistance of the aqueous solution to hydrolysis and compatibility with the alkyd resins to be emulsified are achieved, which allow solvent-free alkyd resins to be emulsified in the form of their melt. However, alkyd resin emulsions prepared by this process can be only used to a limited degree for high-gloss and low-yellowing pigmented topcoats because the emulsifying resin used adversely influences gloss retention and yellowing of the paint under weathering conditions.